Internal combustion engines, including compression ignition engines and spark ignition engines regularly undergo redesign efforts to improve efficiency and enhance fuel economy. Compression-ignition engines and direct-injection spark-ignition engines are gaining in popularity due in part to improved fuel economy, which may exceed 20% improvement compared to a similarly-sized, conventional spark-ignition engine. Compression-ignition engines and direct-injection spark-ignition engines operate with excess air in the combustion process, which is also referred to as operating lean of stoichiometry. An engine that operates lean of stoichiometry can do so without a throttle valve in the air intake manifold. Stoichiometry is an air/fuel ratio at which there is a sufficient amount of oxygen from the air mixed with the fuel to completely oxidize the fuel during combustion. When air can freely flow into the cylinders on an intake stroke of a combustion cycle, less pumping energy is required, leading to a fuel economy benefit. Engines that operate lean of stoichiometry can be classified as heavy-duty diesel, light-duty diesel, and direct-injection gasoline engines. Heavy-duty diesel engines are distinguished from light-duty diesel engines by their application and method for emissions certification. A heavy-duty engine is used in a high-load application, and is typically certified for use using an engine dynamometer, whereas a light-duty engine is used in a passenger vehicle or light truck, and is certified for use on a vehicle dynamometer.
Acceptance of compression-ignition engines and direct-injection spark-ignition engines has been limited due to the inability to comply with increasingly strict emissions regulations. Emissions regulations that are implemented in countries throughout the world include standards for allowable levels of exhaust gas constituents that are output as a result of the combustion process. The primary regulated exhaust gas constituents include hydrocarbons (‘HC’), carbon monoxide (‘CO’), nitrides of oxygen (‘NOx’), and particulate matter (‘PM’). Engine manufacturers meet various emissions regulations by designing engines, engine control systems and exhaust aftertreatment devices to reduce NOx to nitrogen (‘N2’) and oxygen (O2), and oxidize HC, CO, and carbon and organic portions of the PM to water (‘H2O’) and carbon dioxide (‘CO2’). When compression-ignition engines and direct-injection spark ignition engines operate with a fuel charge that is at an air/fuel ratio that is lean of stoichiometry, the result is low engine-out emissions of CO and HC. However, lean operation also typically results in higher levels of engine-out emissions of NOx and PM.
Engine system developers have sought to reduce NOx and PM emissions of compression-ignition engines and direct-injection spark ignition engines using several different aftertreatment devices and control schemes. The aftertreatment devices include, for example, oxidation catalysts, lean NOx catalysts, NOx adsorber catalysts, diesel particulate traps, oxidation and three-way catalysts, and selective catalytic reduction catalysts. The aftertreatment devices are placed in an exhaust gas feedstream and are used in conjunction with engine management control schemes and added hardware to reduce tailpipe emissions below regulated levels.
A NOx adsorber catalyst is an aftertreatment device that is comprised of a ceramic or metal substrate having a washcoat that contains noble metals that are able to catalyze exhaust emissions at elevated temperatures. The noble metals typically include rhodium, platinum, and palladium. The washcoat typically contains barium and other alkali metals that adsorb and store NOx while the engine is operating with excess oxygen. The NOx adsorbed by a NOx adsorber catalyst must be periodically reduced, which is a process wherein NOx is desorbed from the catalyst and then catalyzed. If the NOx adsorber catalyst is not able to periodically reduce the NOx adsorbed, it eventually saturates, leading to breakthrough of NOx emissions. Desorption and catalysis of the NOx requires an exhaust gas feedstream that is rich of stoichiometry, preferably with catalyst bed temperatures above 200° C. The temperature of the exhaust gas feedstream also affects the amount of time that is required to reduce NOx adsorbed by the NOx adsorber catalyst. Currently available NOx adsorber catalysts perform optimally when the temperature of the exhaust gas feedstream is in the range of 350° C. to 450° C. This exhaust gas temperature range is difficult to achieve with a compression-ignition engine or direct-injection spark ignition engine that is operated under low-speed, light load driving conditions.
Reduction of NOx in the NOx adsorber catalyst comprises having the engine management system change the fuel charge from a lean air/fuel ratio to a rich air/fuel ratio for a predetermined amount of time. When the rich exhaust gas enters the NOx adsorber catalyst, the stored NOx is desorbed from the washcoat and reacts with exhaust gases including CO, hydrogen (‘H2’) and HC in the presence of the noble metals to form water (‘H2O’), carbon dioxide (‘CO2’), and nitrogen (‘N2’). The reduction cycle typically must occur regularly during operation of the engine. The engine management system resumes normal engine operation after reduction is complete. The prior art uses the engine management system to switch the fuel charge from a lean air/fuel ratio to a rich air/fuel ratio by reducing overall air intake or adding fuel during combustion. The reduction of air intake to the combustion cycle is accomplished by a combination of throttling, reduction in boost from a turbocharger, and increase in EGR. These methods adversely affect fuel economy, and potentially also affect engine performance.
The performance of a NOx adsorber catalyst is negatively affected by the presence of sulfur in fuel. Sulfur burns in the combustion process to form sulfates (SO2 and SO3). The NOx adsorber catalyst preferably selects and adsorbs sulfates over NOx. The sulfates are not released and reduced during periodic rich air/fuel ratio operation as readily as NOx is released. As a result, adsorbed sulfates reduce the capacity of the NOx adsorber to adsorb NOx.
Desulfation of the NOx adsorber catalyst requires a periodic excursion of the exhaust gas to high temperatures (catalyst bed temperatures of 650° C.) at a rich air/fuel ratio for an extended period of time, typically requiring minutes of operation. Desulfation must occur periodically over the life of the engine, typically every 3,000 to 10,000 miles or an equivalent number of hours of engine operation, depending on the level of sulfur in the fuel, fuel consumption of the engine, and the NOx storage capacity of the NOx adsorber catalyst.
The prior art also reduces NOx emissions using a selective catalytic reduction catalyst (‘SCR catalyst’). The SCR catalyst is an aftertreatment device that is comprised of a catalyst and a system that is operable to inject material such as ammonia (‘NH3’) into the exhaust gas feedstream ahead of the catalyst to reduce the NOx adsorbed by the catalyst. The SCR catalyst consists of a substrate and a washcoat containing noble metals that is capable of creating conditions for reduction of NOx by NH3. This also includes the use of urea, which when decomposed in the exhaust, creates NH3.
The prior art uses the SCR catalyst and operates the engine at a lean condition while injecting NH3 or urea. The NH3 or urea selectively combines with NOx to form N2 and H2O in the presence of the catalyst. The NH3 material must be periodically replenished. Use of urea or other sources of NH3 requires precise control of injection. Overinjection may cause a release of NH3 into the atmosphere, and underinjection may result in inadequate emissions reduction. The additional hardware to inject NH3 must be diagnosed by an onboard diagnostic system, and potentially increases warranty.
A diesel particulate trap is an aftertreatment device that is typically comprised of a ceramic wall flow substrate having a washcoat that is operable to trap, or filter, carbon particulate matter. It may also contain noble metals, typically including platinum. The diesel particulate trap removes PM from the exhaust gas feedstream by passing the feedstream through pores in walls of the ceramic wall flow substrate. The diesel particulate trap must be periodically purged to prevent plugging and associated engine operating problems.
The prior art purges the diesel particulate trap by controlling the engine management system so the exhaust gas passing through the filter is at a high temperature (typically exhaust gas temperatures of 500° C. to 600° C.). This is significantly higher temperature than typically obtained at low speed, light load driving conditions. The high exhaust gas temperature combines with excess oxygen in the exhaust to oxidize carbon and organic PM and form CO2. The purge cycle must occur periodically over the life of the engine, ranging from as frequently as every 100-500 miles for an engine operating at light load and low exhaust gas temperatures. Engines that typically operate at higher load conditions, and therefore at higher exhaust gas temperature, have to purge the diesel particulate trap less frequently. The engine management system performs purge of a diesel particulate trap by maintaining the fuel charge at a lean air/fuel ratio, and generating hot operation by injecting additional fuel into the combustion chamber at the end of the combustion cycle. This creates a combustible mixture in the diesel particulate trap, or in an oxidation catalyst that precedes the diesel particulate trap, such that heat generated by combustion of the combustible mixture enhances oxidization of the stored PM and forms CO2. The engine management system resumes normal engine operation after purging is complete. This method adversely affects fuel economy and potentially also affects engine performance. This system may also increase HC emissions of the engine.
The prior art enhances purging of the diesel particulate trap by introducing a catalyst in the fuel system during normal operation so it accumulates and mixes with the trapped particulate matter. A tank containing a catalyst, typically liquid cerium in solution, is carried within the vehicle and selectively added to the fuel to accomplish mixing with the trapped particulate matter. The catalyst acts to reduce the temperature necessary for combustion of PM in the trap. The catalyst material must be periodically replenished, and the trap must also be purged of the catalyst material to prevent excess flow restriction.
Other aftertreatment devices used with engines that operate lean of stoichiometry include oxidation catalysts and three-way catalysts. These catalysts are comprised of a ceramic or metal substrate having a washcoat that contains noble metals. The noble metals for an oxidation catalyst typically include platinum or palladium. The noble metals for a three-way catalyst typically include rhodium, platinum, and palladium. These devices typically operate at or about stoichiometry.
Many of the above-described aftertreatment devices and systems require elevated exhaust gas temperatures for effective operation. The prior art has sought to increase exhaust gas temperatures by changing engine operation. This includes equipping a compression-ignition engine with a throttle and partially closing the throttle to reduce the amount of excess air reaching the engine. This acts to increase combustion temperatures and exhaust gas temperatures. The prior art has also increased engine exhaust gas re-circulation (EGR), changed timing of fuel injection relative to crank position, quantity of fuel injected, and changed valve timing to increase exhaust gas temperature. Attempts have also been made to increase exhaust gas temperature by directly oxidizing fuel in the exhaust, and by electrically heating catalytic devices. These approaches can increase exhaust gas temperature, but have the penalties of reducing fuel economy and increasing engine-out PM levels.
Exhaust gas temperatures can be increased using advanced engine control hardware and technology. Manufacturers of spark ignition engines have been implementing cylinder deactivation systems to broaden the dynamic operating range of a specific engine configuration, leading to improvements in engine efficiency and vehicle fuel economy. Cylinder deactivation is currently being implemented on throttled spark-ignition engines such as are commonly used in cars and light trucks. These engines obtain a relatively large fuel economy benefit (8% to 25%) from use of cylinder deactivation when operating at low power demand levels. This benefit is a result of reduced engine pumping losses obtained during operation of the cylinder deactivation system. Cylinder deactivation is currently not used on compression-ignition or direct-injection spark-ignition engines primarily because there is little efficiency gain or fuel economy benefit for these engines due to the fact that the engines normally operate in an unthrottled mode with excess air. Therefore compression-ignition or direct-injection spark-ignition engines generally have low pumping losses.
Pumping losses comprise the energy required to pump air from an intake system, through the engine, and out of the exhaust system. Pumping losses reduce the total amount of energy that the engine can translate into work. A typical multi-cylinder engine has an engine block with multiple cylinders, and a piston in each cylinder that is operably attached to a crankshaft. There is also at least one intake valve and at least one exhaust valve that allow passage of air into and out of each cylinder. A combustion chamber is formed inside each cylinder. The typical engine operates on a four-stroke cycle that sequentially includes an air intake stroke, a compression stroke, a power stroke, and an exhaust stroke. During the air intake stroke the piston moves away from the intake and exhaust valves and creates a negative pressure in the combustion chamber. Pumping loss during air intake is due to the negative pressure in the combustion chamber that is working against the movement of the piston away from the intake and exhaust valves. During the exhaust stroke the piston toward the intake and exhaust valves and creates a positive pressure in the combustion chamber. Pumping loss during exhaust is due to the positive pressure in the combustion chamber that is working against the movement of the piston toward the intake and exhaust valves.
When a cylinder is active, the pumping loss during air intake is a measure of a restriction in the air intake system and includes air flow restrictions between the combustion chamber and the outside air, and includes the intake valves, the intake manifold, any throttle device, and air cleaning device. The pumping loss during exhaust is a measure of a restriction in the exhaust system and includes airflow restrictions between the combustion chamber and the outside air, and includes the exhaust valves, the exhaust manifold, exhaust pipes, mufflers, resonators, and any exhaust aftertreatment devices, including catalytic converters and particulate traps. On engines employing an air throttle device, pumping losses are great during period of low power demand. This is caused by a large airflow restriction, and corresponding negative pressure, into the combustion chamber when the throttle device is only partially opened. Internal combustion engines and pumping loss measurement and description is well known to one skilled in the art.
When one or more cylinders are deactivated, there is a reduced demand in the intake system for incoming air. The reduced demand for incoming air results in less negative pressure being created in each combustion chamber during the intake stroke. On engines employing an air throttle device, the effect of the reduced demand for incoming air is more pronounced, in terms of the effect of the restriction on airflow. This results in less pumping loss through the engine, resulting in higher translation of energy into power, or work.
A cylinder deactivation system operates by collapsing the opening mechanism of the inlet and outlet valves of each deactivated cylinder, so the valves each remain in a closed position. Fuel delivery is also discontinued to each deactivated cylinder. This action stops the flow of air and fuel to each deactivated cylinder. When cylinder deactivation occurs, an engine controller operates the active cylinders with greater amount of fuel and air to meet the extant power demands of the engine and vehicle. The active cylinders each operate with greater airflow, reducing pumping losses due to throttling of the air intake, and improving fuel efficiency in the active cylinders. The active cylinders also achieve higher operating temperatures.
Hence, there is a need to provide an engine control system and exhaust aftertreatment system for engines that operate at an air/fuel ratio that is primarily lean of stoichiometry that is able to meet emissions regulations by effectively regenerating various components of the aftertreatment system, without adversely affecting fuel economy and engine performance. There is also a need to control an engine that operates at an air/fuel ratio that is primarily lean of stoichiometry such that the exhaust gas feedstream periodically operates at a rich air/fuel ratio, within a range of temperatures, for a sufficient amount of time to regenerate the aftertreatment system with a minimal effect upon fuel economy and performance. There is a further need to control an engine that operates at an air/fuel ratio that is primarily lean of stoichiometry such that the exhaust gas feedstream operates at a lean air/fuel ratio and within a range of temperatures for an amount of time sufficient to desulfate the aftertreatment system. This includes systems for use on compression-ignition engines and direct-injection spark-ignition engines.